1. Field of the Invention
The present invention relates to a manufacture method for a ZnO substrate, and more particularly to a method of manufacturing a ZnO substrate from a ZnO crystal formed by a hydrothermal synthesis method.
2. Description of the Related Art
Description will be made on a ZnO substrate manufacture method disclosed in JP-A-2004-296821. First a ZnO single crystal substrate is formed by using a hydrothermal synthesis method. A buffer layer made of ZnO— contained compound is formed on the substrate. An element layer made of ZnO is formed on the buffer layer. Interface impurities may exist excessively on the principal surface and nearby surfaces of the substrate formed by the hydrothermal synthesis method. It is possible to effectively suppress growth of lattice defect and dislocation to be caused by interface impurities.
ZnO crystal formed by the hydrothermal synthesis method is sliced and polished in a wafer form, and thereafter subjected to heat treatment in a crucible of polycrystalline ZnO. This heat treatment removes strain in a surface layer formed during surface polishing and further planarizes the surface. If the principal surface of the substrate is a −c face (O-polar face), the substrate is planarized to such a degree that steps having height of one atom layer can be observed. In this specification, the “principal surface” means one of both surfaces of a substrate on which a functional element such as a light emitting device is formed.
The present inventors have found that if the principal surface of a substrate is a +c face (Zn-polar face), step bunching, that is, steps having a height corresponding to a plurality of molecular layers occurs and the surface has large irregularity. This may be ascribed to a higher dissociation pressure at the +c face than that at the −c face.
The present inventors have found also that the electrical characteristics of a ZnO film are shifted from desired characteristics, if the ZnO film is formed by metal organic chemical vapor deposition (MOCVD) or the like, on a ZnO substrate formed by the hydrothermal synthesis method. It has been found that it is difficult to form desired electrical characteristics, particularly when a ZnO film of n-type conductivity type is formed. This reason will be described below.
FIG. 8 shows the measurement results of impurity concentrations contained in a ZnO substrate formed by the hydrothermal synthesis method and subjected to heat treatment, the impurity concentrations being measured by a secondary ion mass spectrometry method. The abscissa represents a substrate depth in the unit of “μm” and the ordinate represents an impurity concentration in the unit of “cm−3”. It can be seen that a relatively large number of Li atoms are contained in a surface layer shallower than a depth of 0.4 μm. Li atoms contained in the substrate result from use of lithium hydroxide (LiOH) as solvent during crystal growth by the hydrothermal synthesis method.
As a Li atom is captured in a ZnO crystal at a Zn lattice position, the Li atom functions as a p-type dopant, and as a Li atom is captured at an inter-lattice position, the Li atom functions as an n-type dopant. Conductivity of ZnO is therefore influenced. As a ZnO film is formed on the ZnO substrate, Li atoms in the substrate are diffused into the ZnO film. Since the electrical characteristics of the ZnO film are influenced by the diffused Li atoms, it becomes difficult to obtain desired electrical characteristics.
As shown in FIG. 8, it can be considered that Li atoms captured in the ZnO substrate diffuse toward the surface and a Li concentration in the surface layer increases. In order to control the electrical characteristics of the ZnO film formed on the substrate to have desired electrical characteristics, it is necessary to remove Li atoms contained in the substrate prior to forming the ZnO film.